A pigment is generally insoluble in solvents such as an organic solvent and water. When a pigment as a colorant is used in an ink, a coating composition, plastics or a coloring image-forming material such as a liquid developer, it is often dispersed in an organic solvent or water as required for the particular use. Recently, in particular, it is strongly desired to form a water-based pigment in view of environmental problems or influences on human bodies, and a pigment is required to have stable dispersibility in water. For dispersing a pigment in water, conventionally, the pigment surface is modified with a surfactant, a dispersant or a dispersing resin to form a water-based ink or a water-based coating composition. However, most of organic pigments are lipophilic and have a problem that it is difficult to disperse them in water. That is, to maintain the dispersion state of a pigment in a water-based vehicle, a dispersant or a surfactant is selected for each pigment, dispersing conditions are modified, or pre-dispersing is required. Further, when a pigment is contained in an inkjet ink, often the dispersed ink coagulates in a nozzle portion, and it is difficult to re-disperse the coagulated ink. When a pigment is contained in a coating composition, for example, the problem is a phenomenon of pigment particles precipitating due to coagulation.
Attempts have been therefore made in various ways to improve a pigment in dispersion stability. One attempt, which has been already proposed, is an improvement in the method of imparting an organic pigment with hydrophilic nature. The hydrophilic nature is imparted by introducing a chemically hydrophilic group into each molecule of an organic pigment thereby to improve the organic pigment compatibility with a hydrophilic vehicle, so that the pigment is improved in dispersion stability.
However, when the number of the introduced hydrophilic groups is large, the above treatment converts an organic pigment to a water-soluble dye, and a phenomenon takes place wherein the ink transferred to a receptor sheet is poor in waterfastness and lightfastness. That is, there is a problem in the performance of the ink. Further, when the number of the introduced hydrophilic groups is small, the effect on the dispersion stability is extremely low. In the above method of introducing a hydrophilic group into each molecule of a pigment, it is difficult to control the introduction of the hydrophilic group, and it is very difficult to make the forming of a hydrophilic pigment and the fastness of an ink consist with each other.
On the other hand, various methods of chemically directly treating a pigment surface have been proposed. Among them is a method in which a pigment surface is treated with a fluorine gas (Kagaku, Vol. 46, No. 9, 1991).
It is disclosed that, in the above method, the surface of each particle is imparted with hydrophilic nature by ionizing the surface of each carbon surface with the fluorine gas whereby carbon having excellent dispersibility is obtained. However, the ionization of carbon particle surfaces causes a problem with the stability and performance when it is stored in a powder state, and another defect of the above method is that the carbon particles are unstable with regard to a pH of a dispersion and a change in temperature.
Further, there have been attempts to obtain a carbon black having excellent dispersibility in a water-based vehicle. That is, a sulfonic acid group is chemically introduced onto the surface of each carbon black particle to impart the carbon black with hydrophilic nature (Cabot Corporation: U.S. Pat. Nos. 3,442,679 and 3,519,452, Huber: U.S. Pat. No. 3,528,840). For example, one of the above attempts is directed to a treatment method in which carbon black is reacted with fuming sulfuric acid to sulfonate the carbon black surface. When this treatment method is applied to an organic pigment, the organic pigment is dissolved or violently decomposed because of the fuming sulfuric acid. Even when the sulfonation takes place, it takes place on a molecular level and rarely takes place on the surface, so that the treatment effect differs between carbon black and an organic pigment. Further, since the sulfonation entails a morphological change such as crystal transition or crystal growth, the surface reaction cannot be controlled, and the sulfonation therefore cannot be applied to an organic pigment. In another attempt, ammonium sulfate or ammonium sulfite is mixed with carbon black and the mixture is heated to decompose the ammonium compound and to eliminate ammonia gas so that the carbon surface is sulfonated. When this treatment is applied to an organic pigment, the oxidation treatment at a high temperature entails violent decomposition, the reaction proceeds non-uniformly, and the reaction efficiency is poor. The above treatment therefore cannot be applied to an organic pigment.
In addition to the above methods, treatments in a gaseous phase such as ozone treatment and plasma treatment are being studied. With regard to the plasma treatment, studies are under way using an inert gas such as He or Ar or a reactive gas such as H.sub.2, O.sub.2, N.sub.2, CO, CO.sub.2, H.sub.2 O or CF.sub.4. Of these, the use of low-temperature oxygen plasma treatment (IHARA Tatsuhiko: Shikizai (Coloring Materials), 54,531, 1981) and the treatment with ammonia plasma (S. IKEDA: J. Coat. Technol., 60,765, 1988) are proposed as treatments having effects on organic pigments. The treatment with oxygen plasma cannot be used for some pigments due to a color change caused by oxidation, and the treatment with ammonia plasma is employed for improving the dispersibility and the weatherability. These plasma treatments are advantageous in dry treatment, while they are not always sufficient concerning the uniformity in treatment and an effect on dispersibility. None of these are practically used.
As discussed above, chemical treatments of organic pigments have been attempted in various ways, while there has been found no treatment method which has a practical effect on the dispersibility in a water-based vehicle.
A variety of methods for producing pigment dispersions have been proposed. In most of the methods, a surfactant, a dispersant, a dispersing resin or a pigment derivative is used, and a dispersing machine or a milling machine is used the combination of ingredients. These methods are advantageous in that pigments dispersions can be relatively easily produced, while it is required to change a surfactant, a dispersant or a pigment derivative depending upon the kind of a pigment. Further, for some uses, there are many points to be noted, such as compatibility and a mutual effect between a resin and a dispersant, and a decrease in the dispersibility of a pigment in a solvent caused by dissolving a surfactant or a dispersing resin. The above methods therefore have problems which remain to be solved for producing compositions such as an ink, a coating composition, etc.
On the other hand, concerning chemically surface-treated pigments, a surface-treated carbon black is practically used for dyeing a skin and leather or in an inkjet ink. However, when an organic pigment is treated only by a conventional method such as plasma treatment, it is difficult to maintain the dispersion stability of the organic pigment, and almost in no case, a pigment dispersion has been produced without an aid such as a surfactant, a dispersant or a dispersing resin.
An inkjet recording liquid prepared by dissolving a water-soluble dye such as an acidic dye, a direct dye or a basic dye in a glycol-containing solvent and water is often used as is disclosed in JP-A-53-61412, JP-A-54-89811 and JP-A-55-65269. The water-soluble dye is generally selected from those having a high solubility in water for attaining the stability of the recording liquid. As a result, there is a problem that an inkjet printing (recorded product) has poor waterfastness so that a dye of a record portion feathers when water is spilled on it.
For overcoming the above poor waterfastness, attempts have been made to alter the structure of a dye or to prepare a recording liquid having a high basicity, as is disclosed in JP-A-56-57862. Further, attempts have been made to utilize a reaction between a receptor sheet and a recording liquid, as is disclosed in JP-A-50-49004, JP-A-57-36692, JP-A-59-20696 and JP-A-59-146889.
The above methods have remarkable effects when specific recording sheets are used, while these methods are limited to specific recording sheets and are therefore insufficient as a method applicable to general sheets. When a recording sheet other than the specific sheets is used, a recording liquid containing a water-soluble dye often fails to give a recorded product having sufficient waterfastness.
As a recording liquid having excellent waterfastness, there is a recording liquid obtained by dissolving or dispersing an oil-soluble dye in a solvent having a high boiling point or a recording liquid obtained by dissolving an oil-soluble dye in a volatile solvent. However, these recording liquids have problems concerning the odor and disposal of the solvents and are environmentally undesirable. Further, when a large volume of recorded products are made, the problem is that it is required to recover or recycle the solvent as required depending upon a place where a printing machine is installed.
For improving a recorded product in waterfastness, it is therefore under way to develop a recording liquid which is a dispersion of a pigment in a water-based system.
However, unlike a dye, it is very difficult to disperse a pigment in the form of fine particles and maintain the dispersion of a pigment in a stable state. On the other hand, with an increase in the resolution of a printer for use with an inkjet recording liquid, the diameter of a nozzle of the printer decreases. It is therefore required to decrease the diameter of a pigment so as to form fine particles. In image-forming on a transparent substrate of an overhead projector, etc., it is required to attain transparency equivalent to that of dye, and a pigment is also required to be finely milled in view of the color development of the pigment.